An elevator installation with a shaft in which at least two cars are movable along a common travel path is known from DE 1 562 848 B1. In the known elevator installation the cars each comprise a safety brake device with which a control unit, a drive and a brake are associated. In addition, a shaft information system, which is connected with an electrical safety device, for determination of the positions and speeds of the cars is provided. In that case spacing sensors are provided which serve for determination of the spacing adopted by a specific car from an adjacent car or a travel path end and preferably also from a predetermined shaft point are provided, wherein the spacing sensors are connected with the safety device.
In order to trigger an emergency stop in a case of impermissible approach of two cars the triggering of at least one safety brake device is additionally provided, the triggering taking place mechanically. In the case of an intentional mutual approach of the two cars at very low speed, for example during an inspection or maintenance journey, however, no safety brake device is triggered. If, however, the cars have a higher speed then it is ensured by provision of a correspondingly high minimum spacing value that in the case of an impermissible approach a collision can be reliably prevented by triggering the respective safety brake device. The safety device can then comprise a determining unit which determines a speed-dependent minimum spacing.
The elevator installation known from EP 1 562 848 B1 has the disadvantage that large variations with respect to the braking travel arise, since the preset normal force generates, due to varying coefficients of friction, varying braking forces and these in turn produce different degrees of retardation depending on the respective load state of the respective car. At high car speeds these physical boundary conditions lead to very long stopping paths, since the braking travel increases at least approximately with the square of car speed.